Electrolytic method for enlarging the bore of a tubular workpiece



Oct. 13, 1970 J. zuBAK ETAL 3,533,926

ELECTROLYTIC METHOD FOR ENLARGING THE BORE OF A TUBULAR WORKPIECE Filed Sept. 25, 1967 Ff/ZW I INVENTORS 7% United States Patent O US. Cl. 204-143 4 Claims ABSTRACT OF THE DISCLOSURE A conductive tubular workpiece is held between two hollow work holders which communicate with the workpiece bore for electrolyte flow from one work holder to the other through the workpiece. A conductive pull rod passing through the one work holder into the bore of the workpiece carries an electrode in the bore. Electric current is passed from the workpiece through the electrolyte, the electrode, and the pull rod so that the bore surface anodically dissolves in the electrolyte. As the bore surface is being electrolytically dissolved, the electrode is moved through the bore in a direction counter to the fiow of the electrolyte so that the relative movement of the electrolyte and the electrode assembly maintains the surface of the bore free from contact. A short circuit is prevented by a spacer mounted on the leading or trailing end of the electrode and projecting toward the workpiece.

BACKGROUND OF THE INVENTION This invention relates to the electrolytic machining of workpieces, and particularly to an electrolytic method and apparatus for enlarging the bore of a tubular workpiece.

The invention is more specifically concerned with the electrolytic broaching or polishing of bores which have a length many times greater than their diameter so that an electrode cannot be guided with the necessary precision through the bore by a rod which moves the rod and provides a connection with the negative terminal of a direct current source as is conventional in the broaching of relatively short or shallow openings.

The primary objects of the invention are the provision of an electrolytic method for precisely broaching or polishing a long and narrow bore in a tubular workpiece, and the provision of apparatus for carrying out the method.

SUMMARY OF THE INVENTION In one of its aspects, the invention thus resides in a method of electrolytically working a tubular workpiece in which an electrolyte is introduced into one end of the bore in the workpiece under a predetermined pressure and is withdrawn from the other end of the bore at a lower pressure so that a body of electrolyte is maintained in the bore while an electric potential is maintained between respective surfaces of the workpiece and of an electrode which is being moved through the bore and located in a portion of the same intermediate the afore-mentioned ends. The difference between the predetermined supply pressure of the electrolyte and its lower draining or return pressure must be sufiicient to maintain an annular gap in the bore between the surfaces of the electrode and the workpiece so that the electrolyte can flow through the gap.

Apparatus of the invention for performing this method and variations thereof includes first and second hollow work holders and means which secure respective spaced 3,533,926 Patented Oct. 13, 1970 portions of an elongated tubular workpiece to the work holders in such a manner that the bore of the workpiece communicates with the hollow interior of each work holder through an opening in the latter, the first work holder being formed with a passage therethrough aligned with the opening. An elongated pulling member is longitudinally mova'bly received in the passage in such a manner that one portion of the pulling member projects from the passage, and another portion projects from the opening of the first work holder.

An electrode is mounted on the last-mentioned portion of the pulling member. The pulling member, the first work holder, or both are provided with insulating means which electrically insulate the electrode from the workpiece. A hydraulic circuit supplies electrolyte under pressure to one of the work holders, preferably the first work holder, and a return is provided for the electrolyte from the other work holder.

Other features, additional objects, and many of the attendant advantages of this invention will readily be appreciated as the same becomes better understood from the following description of preferred embodiments when considered with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an arrangement of the invention for enlarging or polishing the bore of a tubular workpiece, in a perspective view partly insection; and

FIG. 2 shows a modification of the arrangement of FIG. 1 in a similar, but fragmentary view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing in detail, and initially to FIG. 1, there is seen a tubular, metallic workpiece whose bore 21 is to be enlarged by current flow between the workpiece surface in the bore 21 and an electrode 22 coaxially received in the bore 21, the workpiece being the anode in an electrolytic circuit and being supplied with current by a contact clamp 23 engaging the outer surface of the workpiece 20. The necessary spacing between the electrode 22 and the workpiece 20 is maintained by a spider shaped spacer 24 attached to the trailing axial end of the electrode 22 and consisting of hard rubber.

The workpiece 20 is only partly shown in the drawings, and it will be understood that its actual length is at least twenty times the diameter of the enlarged bore therein and may be much greater. Its axial ends are threadedly received in sealing engagement in fiat end faces of two hollow, approximately cylindrical work holders 34, 34- of plastic, such as non-plasticized polyvinyl chloride, or hard rubber.

The other fiat end face of the work holder 34 has an axial passage 26 aligned with the workpiece 20. A packing 25 in the passage provides a seal about a pull rod 28 which is coaxially attached to the electrode 22 and is covered with a polytetrafluoroethylene coating 27 over most of its length. The electrode 22 flares axially from one end of the rod 28 to the spacer 24. The other end of the rod 28 is attached by a pivot pin 29 to a cup-shaped piston 30 slidably received in a cylinder 31, Suction applied to the cylinder draws the piston 30 axially away from the work holder 34 during the operation of the illus trated apparatus, as indicated by an arrow S, and thereby draws the electrode 22 axially through the 'work piece 20 in a direction from the work holder 34' to the work holder 34. I

The latter is equipped with a nipple 37 in its curved surface. A shut-off valve 38 connects the nipple 37 with an electrolyte supply pipe 40. The work holder 34' is similarly equipped with a nipple 35 which is connected to an electrolyte return or drain pipe 39 by a throttling valve 36. Electrolyte is driven from the supply pipe 40 through the work holder 34, the work piece 20, and the work holder 34' into the drain pipe 39 by a hydraulic electrolyte circuit including a sump tank (not shown) into which the pipe 39 discharges the drained electrolyte, and by a non-illustrated pump which draws electrolyte from the tank and discharges it into the supply pipe 40 at a rate suflicient to maintain a gage pressure of 30 to 300 lbs. per square inch in the work holder 34.

The cylinder 31 is conductively connected to the negative terminal of a rectifier 32 whose positive terminal is connected to the contact clamp '23. The piston 30 transmits current from the cylinder 31 to the rod 28 which is conductively connected to the electrode 22.

The arrangement illustrated is operated as follows:

The original bore '21 of the work piece 22 is enlarged by mechanical tools at one end of the work piece over a length and to a cross section sufficient to receive the spacer 24 with a clearance of at least 0.01 millimeter. The electrode 22 is assembled with the coated rod 28 and the spacer 24, as shown in FIG. 1 and the rod 28 is pushed through the workpiece until the spacer 24 is received in the previously enlarged portion of the bore 21.

The work holders 34, 34 are next threadedly attached to the two ends of the workpiece 20, the rod 28 being passed through the packing 25 in the passage 26 of the work holder 34. The piston 30 is attached to the projecting bare end of the rod 28 by means of the pin 29 and inserted in the cylinder 31, and the pneumatic, hydraulic, and electrical circuits are established.

When an electrolyte such as aqueous sodium chloride solution is pumped through the workpiece 20 in a direction from the work holder 34 to the work holder 34' as indicated by arrows 41, an electrical potential is maintained between the workpiece and the electrode 22, and the electrode is slowly moved by vacuum in the cylinder 31 as indicated by the arrow S, the internal wall of the workpiece 20 is electrolytically dissolved while hydrogen is liberated at the electrode 22 and is carried away by the rapidly flowing electrolyte together with the chloride of the metal which constitutes the workpiece 20. The axial Position of the electrode 22 is maintained by the spacer 24, and the electrode advances toward the unworked portion of the workpiece at a rate which is determined by the balance between the force of suction in the cylinder 31 and the force exerted by the flowing electrolyte on the electrode 22. Since the latter force decreases with the expansion of the bore 21, the cross section of the bore is readily kept uniform if the electrolyte pressure in the work holder 34 and the current are kept reasonably constant.

In the electrode 22 which flares from the leading end towards its trailing end, the configuration of the latter determines the shape of the enlarged bore and also its cross section under otherwise constant conditions. An initially circular cross section of the bore may readily be converted to a rectangular or any desired shape by means of a suitably shaped electrode.

Current flow in a direction opposite to the direction of electrode movement in the workpiece is preferred under most conditions and necessary if the length of the work piece is more than approximately twenty times the diameter of the bore. If the electrolyte is pumped through the workpiece in the opposite direction, the liquid pressure drop in the narrow gap between the electrode 22 and the surface of the bore 21 is so great that adequate electrolyte flow cannot be maintained along the pull rod 28. The flow section of the apparatus cannot be increased in this area by making the pull rod slimmer and thereby reducing the permissible current and increasing the rvoltage drop along the rod if the bore is very long and narrow. If little current is needed, as in some electropolishing operations for which the illustrated apparatus is suitable in an obvious manner, or if the ratio of bore diameter to bore length is relatively great, reverse electrolyte flow 4 may be resorted to without undue difiiculties. The electrolyte pressure necessary in the work holder 34 for electropolishing is normally between 7 and 90 p.s.i.g., and the pressure in the apparatus can be adjusted by suitably setting the valves 36, 38.

If the diameter of the worked bore portion is controlled for a close sliding fit of the spacer 24 by properly setting the liquid pressures in the work holders 34, 34, the vacuum in the cylinder 31, and the electric current, the apparatus shown in FIG. 1 permits long and narrow bores in a straight tubular workpiece to be broached electrolytically to a uniform cross section.

FIG. 2 shows a portion of a modified electrolytic broaching and polishing arrangement whose non-illustrated portions are practically identical with the corresponding parts of the apparatus shown in FIG. 1.

The modified apparatus has a pulling member 28' which is a flexible copper cable coated with polytetra fiuoroethylene to its end which carries a hard rubber spacer 33 and an approximately torpedo-shaped electrode 22' in such a maner that the spacer 33 is mounted on the end of the electrode 22 which leads relative to the electrolyte flow indicated by the arrows 41, and which is conductively attached to the cable 28.

When the electrode 22' is pulled through the workpiece 20 by the cable 28' while a potential is maintained across the electrolyte between the opposite surfaces of the workpiece and of the electrode in the bore 21 of the workpiece, the free end of the electrode which axially overhangs the spacer 33 oscillates transversely of the direction of electrolyte flow, but the amplitude of oscillation is small, and an open annular gap is maintained between the electrode 22' and the inner wall of the workpiece 20 as long as an adequate pressure dififerential is maintained between the associated work holders, not themselves seen in FIG. 2. If the failure of the hydraulic circuit should re duce the pressure differential to an inadequate value, a short circuit between the electrode 22' and the workpiece 20 is prevented by the spacer 33 which normally is separated rather widely from the workpiece. The spacer 33 thus does not have the guiding function described above with reference to the spacer 24.

The flexibility of the cable 28 permits the modified apparatus to be employed in the electrolytic working of long and narrow bores which are not straight.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the invention, and that it is intended to cover all changes and variations of the examples of the invention herein chosen for the purpose of the disclosure which do not constitute departures from the spirit and scope of the appended claims.

We claim:

1. A method for anodically treating the bore of an elongated tubular workpiece, comprising, securing each end of said tubular workpiece within a hollow work holder, the interior of which communicates with said bore; providing an electrode assembly including an electrode having tapered leading and trailing end portions, and an outside diameter less than the diameter of the bore, conductive pulling means connected at one end of said electrode having its other end extending outwardly of one of said workholders, said pulling means being electrically insulated from said bore, and a radially extending insulator located at said leading end having a diameter less than said bore; applying an electric current across said tubular workpiece and said pulling means to make said workpiece predominantly anodic; introducing electrolyte into one workholder at a predetermined pressure, and withdrawing said electrolyte from the other of said workholder, at a pressure lower than said predetermined pressure; and simultaneously moving said electrode assembly counter to the direction of flow of the electrolyte, whereby the relative movement of the electrolyte and the electrode assembly maintains the electrode assembly from the contact with the bore.

2. The method of claim 1 wherein the electrode is torpedo-shaped.

3. The method of claim 1 wherein said electrode is connected to the negative pole of the working current and the workpiece is connected to the positive pole.

4. The method of claim 1 wherein said electrode assembly is maintained concentrically within the bore.

6 References Cited UNITED STATES PATENTS 2,764,540 9/1956 Farin et a1 204-1405 3,223,600 12/1965 AnderSsOn 204--26 ROBERT K. MIHALEK, Primary Examiner US. Cl. X.R. 

